<form class="form-horizontal">
    <fieldset>
        <legend>Site configuration</legend>
        <div class="control-group">
            <label class="control-label">
                Avatar directory
            </label>
            <div class="controls">
                <input type="text" name="site[avatar_directory]" />
            </div>
        </div>
    </fieldset>
</form>